1. Field of the Invention
The present invention relates to active pixel sensor cells and, more particularly, to an active pixel cell with self reset for increased dynamic range.
2. Description of the Related Art.
Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting a pixel of light energy into an electrical signal that represents the intensity of the light energy. In general, CCDs utilize a photogate to convert the light energy into an electrical charge, and a series of electrodes to transfer the charge collected at the photogate to an output sense node.
Although CCDs have many strengths, which include a high sensitivity and fill-factor, CCDs also suffer from a number of weaknesses. Most notable among these weaknesses, which include limited readout rates and dynamic range limitations, is the difficulty in integrating CCDs with CMOS-based microprocessors.
To overcome the limitations of CCD-based imaging circuits, more recent imaging circuits use active pixel sensor cells to convert a pixel of light energy into an electrical signal. With active pixel sensor cells, a conventional photodiode is typically combined with a number of active transistors which, in addition to forming an electrical signal, provide amplification, readout control, and reset control.
FIG. 1 shows an example of a conventional CMOS active pixel sensor cell 10. As shown in FIG. 1, cell 10 includes a photodiode 12 connected between a first intermediate node N.sub.IM1 and ground, and a reset transistor 14 connected between a power supply node N.sub.PS and the first intermediate node N.sub.IM1.
In addition, cell 10 also includes a buffer transistor 16 and a row-select transistor 18. As further shown in FIG. 1, buffer transistor 16 has a drain connected to the power supply node N.sub.PS, a source connected to a second intermediate node N.sub.IM2 and a gate connected to first intermediate node N.sub.IM1, while row-select transistor 18 is connected between the second intermediate node N.sub.IM2 and an output node N.sub.O.
The operation of active pixel sensor cell 10 is performed in three steps: a reset step, where cell 10 is reset from the previous integration cycle; an image integration step, where the light energy is collected and converted into an electrical signal; and a signal readout step, where the signal is read out.
FIGS. 2A-2B show timing diagrams that illustrate the reset, image integration, and readout steps with respect to cell 10. As shown in FIGS. 1 and 2A-2B, the reset step begins by pulsing the gate of reset transistor 14 with a reset voltage V.sub.RT at time t.sub.1. The reset voltage V.sub.RT turns on reset transistor 14 which pulls up the voltage on photodiode 12 and the gate of buffer transistor 16 to an initial integration voltage. The voltage on the source of buffer transistor 16, in turn, is also pulled up to be one threshold voltage drop below the initial integration voltage on the gate of buffer transistor 16 due to the source-follower operation of buffer transistor 16.
Following this, the value of the initial integration voltage (less the threshold voltage drop of buffer transistor 16) is read out by pulsing the gate of row-select transistor 18 with a row-select voltage V.sub.RS at time t2. The row-select voltage V.sub.RS turns on row-select transistor 18 which causes the voltage on the source of buffer transistor 16 to appear on the source of row-select transistor 18. The voltage on the source of row-select transistor 18 is detected by conventional detection circuitry and then stored as a reset value.
Next, during integration, light energy, in the form of photons, strikes photodiode 12, thereby creating a number of electron-hole pairs. Photodiode 12 is designed to limit recombination between the newly formed electron-hole pairs. As a result, the photogenerated holes are attracted to the ground terminal of photodiode 12, while the photogenerated electrons are attracted to the positive terminal of photodiode 12 where each additional electron reduces the voltage on photodiode 12.
Following the image integration period, the final integration voltage on cell 10 is read out by pulsing the gate of row-select transistor 18 with row-select voltage V.sub.RS at time t.sub.3. At this point, the final integration voltage on photodiode 12, less the threshold voltage of buffer transistor 16, is present on the drain of row-select transistor 18. As a result, when row-select transistor 18 is turned on, the voltage on the drain of row-select transistor 18 appears on the source of row-select transistor 18 where the voltage is detected and then stored as a read value.
Thus, at the end of the integration period, a collected photon value which represents the number of photons absorbed by photodiode 12 during the image integration period can be determined by subtracting the read value taken at the end of the integration period from the reset value taken at the beginning of the integration period.
One problem with active pixel sensor cell 10, however, is that imaging systems which utilize an array of active pixel sensor cells suffer from a limited dynamic range. Conventionally, the dynamic range is defined by the maximum number of photons that a cell can collect during an integration period without saturating (exceeding the capacity of) the cell, and the minimum number of photons that a cell can collect during the integration period that can be detected over the noise floor.
Typically, the dynamic range of an active pixel cell is expressed in bits. The quality of the representation of a digital image is determined by bits, which correspond to the number of possible levels or shades of gray in the pixel representation. Usually, at least 6 bits or 64 gray levels are needed to represent an image adequately. Higher-quality imaging systems use 8 bits (256 levels) or even as many as 10 bits (1024 levels) per pixel. For example, the dynamic range of film is limited to approximately 8 bits.
The effect of a limited dynamic range is most pronounced in images that contain both bright-light and low-light sources. In these situations, if the integration period of the array is shortened to the point where none of the bright-light information is lost, i.e., where the number of photons collected by the cells exposed to bright light does not exceed the capacity of the cells during the integration period, then most, if not all, of the low-light information in the cells exposed to low light will be lost (resulting in a black image) because the collected photons will not be distinguishable over the noise floor.
On the other hand, if the integration period of the array is increased to capture the low-light information, i.e., where the number of photons collected by the cells exposed to low light is detectable over the noise floor, then a significant portion of the bright-light information will be lost (resulting in a white image) because the number of photons collected by the cells exposed to bright light will far exceed the capacity of these cells.
One approach to solving the problem of dynamic range is to utilize a non-integrating active pixel sensor cell with a non-linear load device, such as a MOSFET-diode in weak inversion, to obtain a logarithmic response. This approach, however, has a number of drawbacks.
First, the noise in a non-integrating cell is much higher than the noise in a conventional integrating cell (such as cell 10 of FIG. 1). In a conventional integrating cell, the effect of random noise events is averaged over the integration period, while the effect of random noise events in a non-integrating cell can produce substantial distortions. Second, the exact non- linear transfer function of this type of device must be carefully calibrated to avoid variations from cell to cell and due to temperature changes.
Another approach to solving the problem of dynamic range, which is used with CCD systems, is to integrate twice: once with a short exposure and once with a long exposure. For the short exposure, the bright-light information is saved while the low-light information is discarded. Similarly, for the long exposure, the low-light information is saved while the bright-light information is discarded.
The information from the two exposures is then combined to form a composite image. The drawback with this approach, however, is that the resulting image is formed by combining image data from two different periods of time.
Thus, to successfully capture both bright-light and low-light sources in the same image, there is a need for an active pixel cell with self-reset for improved dynamic range.